skillers are grandcharms with the mod +1 to (random skill tree here) skills.
for example, +1 to sorc fire skills.
This means you can get a total of +10 (if you fill inventory) to a skill tree of choice, thus increasing damage (mostly) of your character.
The best skillers are ofcourse the ones you need. But generally, the skillers with best trade value are paladin combat skillers (for hammerdins), druid elemental skillers (windys), necro poison/bone, sorc skillers, and i probably forgot some now anyway...
Skillers can also have various other mods, of which the most popular are +life, 12%faster hit recovery, and sometimes 7%faster run/walk.
For example, finding a Paladin Combatskiller with +45 life, you'd most certainly be rich
Yes, also skillers can be used at level 42. So you would know that if you found a grand charm in act 1 normal, it wouldn't be a skiller. highest life on a skiller is 40, however if you get a grandcharm dropped by baal who drops item levels of 99, you can reroll it (3pgems+skiller) to a 45. Extremely rare.